Electroless deposition of palladium and palladium alloys in the manufacture of printed circuit boards, IC substrates and the like as well as metallisation of semiconductor wafers is an established technique. The palladium or palladium alloy layers are used as barrier layers and/or wire-bondable and solderable finishes.
The type of palladium deposit (pure palladium or palladium alloy) derived by electroless plating depends on the reducing agent employed.
Formic acid, derivatives and salts thereof result in pure palladium deposits. Phosphorous containing reducing agents such as sodium hypophosphite result in palladium-phosphorous alloys. Borane derivatives as reducing agent result in palladium-boron alloy deposits.
Electroless palladium plating bath compositions comprising a source of palladium ions, a nitrogenated complexing agent and a reducing agent selected from formic acid and derivatives thereof are disclosed in U.S. Pat. No. 5,882,736. Such electroless palladium plating bath compositions are suited to deposit pure palladium.
Electroless palladium plating bath compositions comprising a source of palladium ions, a complexing agent comprising phosphonate groups and a reducing agent selected from formaldehyde, a phosphate ion generator, a boron-nitrogen compound, a borohydride, or an alkylamine borane are disclosed in GB 2034 756 A. Such electroless palladium plating bath compositions are suited to deposit either pure palladium or palladium alloys with boron and/or phosphorous.
A plating bath composition for electroplating of palladium and palladium alloys comprising a palladium diammino-dichloro complex, a nitrite salt as conductive salt and 1-hydroxy-ethane-1,1-diphosphonic acid is disclosed in EP 0 757 121 A1.
A plating bath composition for electroplating of palladium and palladium alloys containing palladium as the palladosammine chloride and an alkylene diamine phosphonate is disclosed in U.S. Pat. No. 4,066,517.
An electroless palladium plating bath composition comprising at least one of hypophosphorous acid, phosphorous acid, formic acid, acetic acid, hydrazine, a boron hydride compound, an amine borane compound, and salts thereof as a reducing agent is disclosed in US 2009/0081369 A1.
Palladium deposition from plating bath compositions according to U.S. Pat. No. 5,882,736 in the presence of copper ions in the plating bath is not possible (comparative examples 1).
Palladium and palladium alloys are deposited onto substrates having a metal surface on at least a portion of said substrates. Typical metal surfaces comprise copper, copper alloy, nickel and nickel alloy.
In case of printed circuit boards, IC substrates and the like as well as semiconductor wafers palladium and palladium alloy deposition is disturbed if the electroless plating bath contains copper ions. The plating rate of palladium or palladium alloy deposition is already strongly reduced at 5 ppm or even less copper ions present in the electroless plating bath. Copper ions may be dissolved from the substrate when immersed in an immersion-type palladium plating bath which is often used as an activation method for the metallic surface prior to palladium deposition from an electroless plating bath. In case the copper surface is not completely coated with a palladium layer in an activation step, copper ions are formed when contacting the copper surface of the substrate with an electroless plating bath for deposition of palladium and/or palladium alloys in the next step. During manufacture of electronic components such as printed circuit boards, IC substrates and metallisation of semiconductor wafers copper ions are then enriched in the electroless palladium and/or palladium alloy plating bath and first slow down and then stop palladium and/or palladium alloy deposition completely.